Bit-fields of type other than int?

Question

I have a code which uses bit-fields declared as follows

typedef struct my{
    const char *name;
    uint8_t is_alpha : 1;   
    uint8_t is_hwaccel : 1; 
    uint8_t x_chroma_shift; 
    uint8_t y_chroma_shift; 

} mystr; 

uint8_t is typedef'ed to unsigned char.

Building the code in MS-VS 2008 using this bit fields gives a warning as below:

imgconvert.c(60) : warning C4214: nonstandard extension used : bit-field types other than int.

1. Is there any problems/potential issues in using bit fields of type other than int? Why the warning?
2. Are other than int type bit-fileds they allowed by C99 C language specification?

Answer 1

1] Is there any problems/potential issues in using bit fields of type other than int? Why the warning?

Since bit-fields are low-level, there may be issues with portability if you are using non-standard types. Hence the warning -- note it is still a warning and not an error.

2] Are other than int type bit-fileds they allowed by C99 C language specification?

From the draft of C99:

6.7.2.1 Structure and union specifiers

4 A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed int, unsigned int, or some other implementation-defined type.

Answer 2

Why not use int? The actual implementation of bitfields varies from compiler to compiler. If you want to write portable code, use int. If you want to create a small structure, or a structure of a fixed number of bytes, or a structure where the bits are in a fixed position, don't use bitfields. Create a uint8_t member called something like flags and define macros to use as bitmasks.